cruising catamaran keel design

[emcmia]

I have a P43 Privilege, 25 ft beam, 26000 lbs displacement with fixed keels. Each keel is approximately 10 ft in length and extend about 24 inches below the rounded hull form resulting in a total draft of 48 inches. The keels are essentially flat slabs with virtually no foil shape and average about 6 inches wide, flared into hull and tapered at the bottom to about 4 inch width.

Underway, either under power (twin 56 hp Yanmars) or sail, especially in shallow water and hull speed over 8 knots, the boat tends to squat, progressively until the bow raises and the waterline at the transom (i.e. swim platforms) submerges as much as 12 inches.

I have extended the swim platforms 48 inches which has resulted in adding at least 2 knots to the hull speed. However, the squatting effect has not changed.

At the next haulout, I am considering changing the shape of the keels to a foil shape, similar to a conventional aircraft wing but symetrical on both side and tapering the trailing edges to less than 1 inch. The objective is reduce the negative pressure that I believe is occurring aft of the keels due to the turbulence created by the existing keel shape. That being said, I am concerned by the prospects of increasing the drag due to the increased width that will be inherent with new foil shape.

Your comments and suggestions would be seriously appreciated.

Thanx, in advance.

[tspeer]

I'm not sure changing the cross section of the keel will change the squat. The keels have to produce enough lift to counter the force of the rig, whether they are thin or thick. And that means a pressure difference on each side. Given the extremely low aspect ratio of the keels, the cross section shape may not mean much.

Making the keels deeper may be a better bet. Or adding a centerboard that fits entirely within the keel.

[san.dam]

This is a very strange effect on a catamaran. The turbulence of a vey thick trailing edge under the stern may be a part of the problem. The displacement seems high enough to be another part...
If the aft sinking occurs only in shallow water,perhaps due to the distortion of the Froude law ,when in shallow water(...) Anyway ,is a very good job to refine the trailing edge, and you can get only advantages.